(1) Field of the Invention
The present invention relates to a tone output device and a tone output integrated circuit for outputting tone signals. More particularly, the present invention relates to a technique of parallel execution of control by CPU and tone signal output.
(2) Description of the Related Art
A tone output device executes tone signal output by a tone output unit, in parallel with a system control by CPU. The system control includes specification of data used for tone signal output. Generally, the tone output unit operates at a relatively low frequency clock that is provided for tone signal playback. On the other hand, the CPU needs to operate at a higher frequency clock than the clock for tone output to make full use of its capabilities.
One technique to address the above need is to provide separate oscillators to generate a clock for the tone output unit and a clock for the CPU. Yet, provision of multiple oscillators involves disadvantages of reducing the packaging density on a semiconductor chip and increasing the manufacturing cost.
In view of the above disadvantages, one technique is suggested in which a single oscillator is employed and different clocks are generated from a clock pulse emitted by the single oscillator and the tone output device and the CPU operate at the respective clocks. This technique is disclosed, for example, in JP patent application publication No. 9-198045
Generally, a clock generating circuit is used to generate clocks of different frequencies than a clock emitted by a single oscillator. Examples of clock generating circuits include a PLL (Phase Locked Loop) and a ring oscillator.
However, a clock generated by a clock generating circuit, such as a PLL, involves a relatively large frequency jitter. Operation of the tone output unit in accordance with a clock generated by such a clock generating circuit inevitably reduces the sound quality of tone output.